In the case of, for example, a rod-shaped sushi product (bou-zushi) or the like, vinegared rice is shaped, and the shaped rice section is topped with a sushi material or materials such as boned and prepared sea bream, sockeye salmon or mackerel together with a thin slice of kombu kelp (shiroita kombu) cooked in a broth seasoned with vinegar, sugar and salt, and resultant is shaped into any of various shapes such as the rod-shape. The shaped sushi product is wrapped with a bamboo sheath and placed in a packing bag made of a plastic film and vacuum-packed, the vacuum-packed sushi product is frozen with a brine such as ethanol, or the shaped sushi product is frozen by slow freezing such as air-blast freezing or nitrogen or carbon dioxide freezing, and the frozen shaped sushi product is placed in a packing bag made of a plastic film and vacuum-packed, to thereby prepare a vacuum-packed frozen sushi product.
Since such a packed frozen sushi product prepared by means of brine freezing is vacuum-packed prior to the freezing, it is free from attachment of bacteria and absorption of odor from the coolant gas in a freezing room. Therefore, it is hygienically superior to, for example, a packed frozen sushi product prepared by vacuum-packing a sushi product posterior to freezing of the sushi product by slow freezing such as air-blast freezing, nitrogen or carbon dioxide freezing. In particular, such a packed frozen sushi product frozen by brine freezing can be frozen-stored stably, for example, over a period as long as 3 months or longer when stored at a storage temperature of −18° C., or can be stored for 6 months or longer when stored at a storage temperature of −50° C.
However, a packed frozen sushi product which has been frozen-stored in such a manner is thawed and then served. Methods for thawing such a packed frozen sushi product include natural thawing, running water thawing, steam thawing, hot water thawing and microwave thawing. Of these, microwave thawing is suitable for thawing of a frozen sushi product or the like because of its extremely short thawing time and high thawing yield. For example, in the case of the rod-shaped sushi product as described above, it takes five hours for thawing at room temperature to thaw the rod-shaped sushi product, whereas microwave thawing enables the rod-shaped sushi product to be thawed in 4 minutes. Accordingly, microwave thawing is capable of instantly responding to demands and thus convenient as compared with thawing at room temperature (see Japanese Unexamined Patent Publication No. Hei 8 (1996)-214805, Japanese Unexamined Patent Publication No. Hei 8 (1996)-317764, Japanese Unexamined Patent Publication No. Hei 9 (1997)-47242, Japanese Unexamined Patent Publication No. 2001-275591 and the specification of U.S. Pat. No. 5,861,184).
However, in the case of microwave thawing, calorific values derived from microwave absorption are different between a sushi material or materials and a rice section, and the sushi material or materials and the rice section are also different in quantity. Further, because a surface portion of the sushi product is intensively heated as compared with a center portion, a portion of the sushi material or materials is heated to an undesirably high temperature as compared with the rice section. The sushi material or materials are thereby brought into the so-called cooked state, and the tastes thereof are impaired. Therefore, microwave thawing has not been practically used. Further, in the case of, for example, a rod-shaped sushi product, a rice section is preferably lukewarm, for example, the rice section preferably has temperatures of 15 to 25° C., more preferably 15 to 20° C., whereas a sushi material or materials is preferred to have temperatures lower than those of the rice section, for example, temperatures of 10 to 25° C., more preferably 10 to 20° C. However, when a packed frozen sushi product is thawed by means of a microwave oven, a sushi material or materials are heated to relatively high temperatures to impair the taste of the sushi product. Accordingly, it has been difficult to effect desired thawing by means of a microwave oven. Under such circumstances, with a view to solving the problem that in thawing of a packed frozen sushi product by means of electromagnetic wave heating with a microwave oven or the like, the sushi material or materials of the sushi product are heated to relatively high temperatures to thereby impair taste of the sushi product, the present inventors have proposed a so-called packed frozen sushi product suitable for frozen storage which is prepared by wrapping a sushi product topped with a sushi material or materials with a plastic wrapping film, placing a metal foil over the sushi material or materials, further wrapping the resultant with a plastic wrapping film, placing the wrapped sushi product in a plastic film packing bag for frozen foods, vacuuming and hermetically seal the packing bag to hermetically pack the sushi product, and freezing the resultant in the hermetically packed state in whole by brine-freezing in a unified form.
In the case of such a packed frozen sushi product, where a plastic wrapping film is placed over the sushi material or materials, and a metal foil is placed thereon, and the above-mentioned rice section and the sushi material or materials are together wrapped by a plastic wrapping film, in thawing by means of electromagnetic heating with a microwave oven or the like, the rice section which is covered with no metal foil and exposed to microwave irradiation is heated with microwaves, whereas the sushi material or materials on which the metal foil is disposed and which is thereby shielded from microwaves are left unheated with microwaves and remain substantially frozen. However, water vapor is generated from the rice section heated by the heating of the rice, and the water vapor generated permeates the inside of the plastic wrapping film to heat the contents wrapped in the plastic wrapping film by steaming during the heating in a microwave oven and even after removal of the wrapped sushi product from the microwave oven, thereby bringing the rice to a lukewarm temperature and heating the sushi material or materials to a temperature of approximately 10° C.
The packed frozen sushi product, which is wrapped with a plastic wrapping film with the metal foil placed covering a surface or surfaces of the sushi material or materials to prevent the sushi material or materials from being heated by microwaves as described above, is steamed by the water vapor generated in thawing, and therefore, it is suitable for thawing by means of a microwave oven. However, the packed frozen sushi product which is wrapped with a plastic wrapping film with the metal foil placed on the sushi material or materials requires such a procedure that the metal foil is placed on the sushi material or materials of the frozen sushi product wrapped with the first plastic wrapping film, and the resultant is wrapped with the second plastic wrapping film to electrically isolate the metal foil from the surroundings, with a view to preventing electric discharge from occurring from the ends of the metal foil at the time of microwave irradiation. Further, when the wrapping with the second wrapping film is performed, the metal foil is required to be placed on the sushi material or materials of the frozen sushi product without misalignment, and the sushi product is required to be placed on the plastic wrapping film and wrapped air-tightly. Any of these procedures in the preparation of the packed frozen sushi product wrapped with the plastic wrapping films require carefulness and skill, and thus make a problem that it is difficult to realize mass production. For responding to customers' demands, the packed frozen sushi product wrapped with the plastic wrapping film has a problem because it requires many hands and much time.
It is an object of the present invention to provide a packed frozen sushi product which is capable of solving the problems inherent in the frozen sushi product wrapped with plastic wrapping films and which is thus suitable for thawing by means of a microwave oven.
The present inventors have found that by placing a sushi product such as a rod-shaped sushi product in a box, placing the sushi product in a flexible packing bag together with the box, and vacuuming the packing bag, the flexible packing bag is pushed by external pressure and brought in tight contact with outer and inner surfaces of the box and an exposed outer surface of the sushi product, i.e., an outer surface uncovered with the box and can be sealed in such an internal volume-diminished condition, and by virtue thereof, when the packed sushi product is heated by means of a microwave oven, a sushi material or materials can be heated by steaming.